The increase demand for higher areal recording densities impose increasingly greater demands on longitudinal magnetic recording media in terms of remanent coercivity (Hcr), magnetic remanence (Mr), coercivity squareness (S*), medium noise, i.e., signal-to-medium noise ratio (SMNR), and narrow track recording performance. It is extremely difficult to produce magnetic recording media that satisfies all or most of these requirements.
Longitudinal recording media is structured as a layered material of metal films deposited on a substrate. The recording media typically has one or more underlayers, such as a chromium (Cr) or a Cr alloy film, one or more magnetic layers, such as a cobalt (Co) alloy, and a protective overcoat. The Co alloy magnetic layer typically contains polycrystallites grown on a polycrystal Cr or Cr alloy underlayer. The underlayer, magnetic layer, and protective overcoat are typically deposited by physical vapor deposition techniques (sputtering).
A magnetic material is composed of a number of submicroscopic regions called domains. Each domain contains parallel atomic magnetic moments but the directions of magnetization of different domains are not necessarily parallel. In the absence of an applied magnetic field, adjacent domains may be oriented randomly in any number of several directions, often called the directions of easy magnetization, which depend on the geometry of the crystal. In a relatively unmagnetized material, the magnetization of each domain is essentially cancelled by an equal and opposite magnetic field produced by an adjacent domain. If a magnetic field is applied many of the domains rotate and align parallel to the applied field. Also, the domains most nearly parallel to the direction of the applied field grow in size at the expense of the others. This is called boundary displacement of the domains or domain growth. These aligned domains further increase the local magnetic field causing more domains to rotate and align parallel to the applied field. When the material reaches the point of saturation magnetization, no further domain growth occurs, even if the magnitude of the external magnetic field is increased.
Magnetic properties, such as remanent coercivity (Hcr), remanent magnetization (Mr) and coercive squareness (S*), which are important to the recording performance of the recording media depends in part on the microstructure of the Co alloy magnetic film for a given Co alloy composition. For longitudinal magnetic recording media, the desired crystalline structure of the Co and Co alloys is hexagonal close packed (HCP) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis in the plane of the film. The better the in-plane c-axis crystallographic texture, the higher the remanent coercivity of the Co-alloy magnetic film.
The grain size of the magnetic film also effects the magnetic performance of recording media. Remanent coercivity increases with an increase in grain size, however, the larger the grain size the higher the medium noise level of the recording media, that is, the lower the SMNR. Thus, there exists a need to achieve high remanent coercivities without the increase in medium noise associated with relatively large grain size. To achieve a low noise recording medium, the Co alloy magnetic layer should have fairly uniform and small grain size with grain boundaries that can magnetically isolate neighboring grains. This kind of microstructure and crystallographic texture is normally achieved by manipulating the deposition process, by grooving the substrate surface, or most often by the proper use of a seedlayer and/or one or more underlayers with a preferred crystallographic orientation.
The linear recording density can be increased by increasing the remanent coercivity and/or by decreasing the medium noise of the recording medium. This can be accomplished by producing a magnetic layer with fine, magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording densities, and is attributed in part to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
The manufacture of longitudinal magnetic recording media can at times include applying a seedlayer between the substrate and the underlayer. A seedlayer provides nucleation of a particular crystallographic texture of the underlayer as well as to subsequent layers, including the magnetic layer.
In co-pending U.S. patent application Ser. No. 09/152,326 filed on Sep. 14, 1998, a magnetic recording medium is disclosed comprising a surface oxidized NiAl seedlayer, and sequentially deposited thereon a Cr-containing underlayer, a CoCrTa intermediate layer and a CoCrPtTa magnetic layer. “Seedlayer Induced (002) Crystallographic Texture in NiAl Underlayers,” L.-L. Lee, D. E. Laughlin and D. N. Lambeth, J. Appl. Phys., 79 (8), pp. 49024904 (1996), discloses a MgO seedlayer. “FeAl Underlayers for CoCrPt Thin Film Media,” L.-L. Lee, D. E. Laughlin and D. N. Lambeth, J. Appl. Phys., 81 (8), pp. 4366-4368 (1997), first reported an FeAl underlayer having a B2 structure.
U.S. Pat. No. 6,174,582 discloses a seedlayer containing a refractory metal that promotes a (200) orientation in the Cr underlayer and a (1 1 {overscore (2)} 0) orientation in the magnetic layer. The refractory metal can be selected from tantalum, niobium, vanadium, tungsten, molybdenum, or chromium. Although the thickness of the seedlayer was stated as not critical, the seedlayer should have a preferred thickness of about 50 Å to 300 Å, and a more preferred thickness of about 100 Å to 300 Å.
In order to store as much digital information as possible on a recording medium there is a continuing need for improved areal density magnetic recording media exhibiting high remanent coercivity and high SMNR. The need for lighter, smaller and better performing computers with greater storage density demands higher density recording media. The present invention satisfies these demands with a longitudinal magnetic recording media having high remanent coercivity and low medium noise.